Pigmented organometallic sol

ABSTRACT

A surface treatment, especially for titanium and aluminum alloys, forms a pigmented sol-gel film covalently bonded on the metal surface to produce desired color, gloss, reflectivity, electrical conductivity, emissivity, or a combination thereof usable over a wide temperature range. The coating retains its characteristics and impact resistance following exposures to temperatures at least in the range from −321° F. to 750° F. An aqueous sol containing an organometallic alkoxide containing either Ti or Al and an organosilane with an organic acid catalyst and stabilizer is applied to etched or grit blasted substrates by dipping, spraying, or drenching, to produce bonds in a single application comparable in strength and performance to standard anodize controls. The sol-gel coating may be graded in its ceramic character by adjusting the organosilane component between TEOS and silanes that have more distinctive organic character by virtue of organic ligands attached to the silicon.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application based upon U.S.patent application Ser. No. 10/384,908 which was a divisionalapplication based upon U.S. patent application Ser. No. 09/169,280,filed Oct. 8, 1998, now U.S. Pat. No. 6,605,365, which was acontinuation-in-part application based upon any of the followingapplications:

-   -   U.S. Ser. No. 08/742,168 (now U.S. Pat. No. 5,849,110); U.S.        Ser. No. 08/742,171 (now U.S. Pat. No. 5,958,578);    -   U.S. Ser. No. 08/740,884 (now U.S. Pat. No. 5,869,141); or U.S.        Ser. No. 08/742,170, each of which was filed on Nov. 4, 1996. It        also claims the benefit of U.S. Provisional Patent Application        60/068,715, filed Dec. 23, 1997.

TECHNICAL FIELD

A sol-gel surface coating containing pigment is applied to a substrate,especially to a metal, through a waterborne reactive sol to provide astable oxide surface that results in corrosion resistance and desiredcolor, gloss, reflectivity, electrical conductivity, emissivity, or acombination thereof over a wide range of temperatures.

BACKGROUND OF THE INVENTION

Conversion coatings for titanium, aluminum, or other metals areelectrolytic or chemical films that promote adhesion between the metaland an organic adhesive resin, especially for adhesive bonding.Anodizing is a conventional process for making electrolytic films byimmersing titanium or its alloys in chromic acid or an alkaline earthhydroxide or aluminum in chromic, sulfuric, or phosphoric acid.Anodizing produces a porous, microrough surface into which primer (adilute solution of adhesive) can penetrate. Adhesion results primarilyfrom mechanical interlocking between the rough surface and the primer.Chemical films include either a phosphate-fluoride conversion coating orfilms made with alkaline peroxide or other alkaline etchants fortitanium substrates and Alodine films (i.e., a chromate conversioncoating) for aluminum substrates.

Because they use strong acids or strong bases and toxic materials(especially heavy metals such as chromates), these surface treatmentprocesses are disadvantageous from an environmental viewpoint. Theyrequire significant amounts of water to rinse excess process solutionsfrom the treated parts. The rinse water and spent process solutions mustbe treated to remove dissolved metals prior to their discharge or reuse.Removing the metals generates additional hazardous wastes that arechallenging to cleanup and dispose. Controlling exposure of workers tothe hazardous process solutions during either tank or manual applicationrequires special control and exposure monitoring equipment thatincreases the process cost. They greatly increase the cost of using theconventional wet-chemical processes. A process that will produceadhesive bonds with equivalent strength and environmental durability tothese standard processes without generating significant hazardous wasteswhile eliminating the use of hazardous or toxic materials would greatlyenhance the state-of-the-art. The present invention is one such process.In addition, the process of the present invention can be applied byspraying rather than by immersion. Therefore, it is more readily usedfor field repair and maintenance.

Surface anodizing chemically modifies the surface of a metal to providea controlled oxide surface morphology favorable to receive additionalprotective coatings, such as primers and finish paints. The surfaceoxides function as adhesion coupling agents for holding the paintlacquer, an organic adhesive, or an organic matrix resin, depending onthe application. Anodizing improves adhesion between bonded metals. Italso improves adhesion between the metal and a fiber-reinforcedcomposite in hybrid laminates, like those described in U.S. Pat. Nos4,489,123 or 5,866,272. We incorporate these patents by reference.Structural hybrid laminates have strengths comparable to monolithicmetal, and have better overall properties than the metal because of thecomposite layers. At higher temperatures (like those anticipated forextended supersonic flight), conventional anodized treatments haveinadequate performance in addition to being environmentally unfriendly.The thick oxide layers that anodizing produces become unstable atelevated temperatures. The oxide layer dissolves into the base metal toproduce surface suboxides and an unstable interfacial layer.

Obtaining the proper interface for the organic resin at the surface ofthe metal is an area of concern that has been the focus of considerableresearch. For example, cobalt-based surface treatments for aluminum aredescribed in U.S. Pat. Nos. 5,298,092; 5,378,293; 5,411,606; 5,415,687;5,468,307; 5,472,524; 5,487,949; and 5,551,994. U.S. Pat. No. 4,894,127describes boric acid—sulfuric acid anodizing of aluminum.

Bonding sites on surfaces for binders include covalent bonds, hydrogenbonds, or van der Waals forces. Conventional approaches (anodizing andchromate conversion coating) promote adhesion by producing a highsurface area coating which has both mechanical and physical (Lewisacid-base, dispersion, hydrogen bonding, etc.) interactions with theadhesion primer. An aerospace coupling agent can be used to createstrong covalent bonds between the metal substrate and the organicprimer. The present invention improves adhesion by crating asol-gel-based coating containing a coupling agent on the metal surface.A metal-to-resin gradient occurs through a monolayer of organometalliccoupling agents. Generally we use a mixture of coupling agents. Theorganometallic compounds preferably have zirconium or silicon activemoieties to interact with, react with, or bond to the metal surface.Some mechanical interaction may result from the surface porosity andmicrostructure. The organic portion of the organometallic compoundsusually has a reactive functional group appropriate for covalentlybonding with the adhesive or matrix resin. A preferred sol-gel film ismade from a sol having a mixture of organometallic coupling agents. Onecomponent (usually containing zirconium) bonds covalently with the metalwhile a second component bonds with the resin. Thus, the sol-gel processorients the sol coating having a metal-to-resin gradient on the surface.

The standard anodizing processes, conversion coatings, or oxide surfacepreparations, especially for titanium, are inappropriate to use with newpolyimide adhesives that are promising as adhesives for vehicles thatwill experience extended exposure to hot/wet conditions. At hightemperatures, the solubility of oxygen in titanium is high and the oxidelayer simply dissolves with the oxygen migrating into the base metal.The result is interfacial failure at the metal-adhesive interface. Toalleviate this type of bond failure, the surface oxygen needs to be tiedup in a stronger bond that will not dissociate in bonding or duringoperation of the system. A zirconate-silicate sol coating of the presentinvention is useful at these extended hot/wet conditions because theZr—O bond that forms between the coating and the metal surface is moredurable than a Ti—O bond. The free energy of formation for the metaloxides is such that a Zr—O bond is more stable at high temperatures thana Ti—O or Si—O bond. The higher bond strength of the Zr—O bond preventsdissolution of the oxide layer, so the Zr component in our sol coatingfunctions as an oxygen diffusion barrier. We can use yttrium, cerium, orlanthanum in addition to or as a replacement for the Zr, because theseelements also produce high strength oxide bonds that function as anoxygen diffusion barrier. The high cost of these compounds, however,dictates that they be used sparingly. Therefore, we developed a mixedmetal coating having Zr and Si to produce the desired metal-to-resingradient needed for good adhesion in structural adhesive bonds, hybridlaminates, or paint adhesion applications. Our coating integrates theoxygen diffusion barrier function of the Zr (or its alternatives) withan organosilicate network desirable for superior bonding performance.

The present invention combines pigments with the sol-gel corrosioninhibitor thin film, to overcome many of the shortcomings of paint.Paints are commonly used to protect a surface and to provide color,gloss, reflectivity, or the like on a substrate. Paints generallydisperse metal or ceramic pigments and a binder in a water or organicvehicle to form a film when dried on a surface. Typically, the binder isan organic resin. Paints generally have application only at relativelylow temperatures. They can be difficult to apply uniformly. They arerelatively heavy and are expensive to repair. Extreme environments, suchas high temperatures or space environments with high ultraviolet, atomicoxygen, and particle exposure, degrade typical organic resins. Thepresent invention combines pigments with sol-gel binder thin films togive greater performance and durability under extreme conditions.

SUMMARY OF THE INVENTION

The present invention is a sol for coating metal surfaces and compositesubstrates, especially aluminum or titanium alloys, to produce a sol-gelfilm, generally containing pigment, as a surface coating having suitableappearance and substrate protection qualities, including color, gloss,reflectivity, electrical conductivity, emissivity, or a combinationthereof. The sol-gel film or sol coating also provides corrosionresistance to a limited degree and can promote adhesion through a hybridorganometallic coupling agent at the metal surface. Our preferred solprovides high temperature surface stability.

We use a sol to produce the sol-gel film on the surface. The sol ispreferably a dilute solution of an organometallic compound, such astetra-i-propoxyzirconium, tetra-n-propoxyzirconium, an aluminumalkoxide, a titanium alkoxide, or a combination thereof, and anorganosilane coupling agent, such as 3-glycidoxypropyltrimethoxysilanefor epoxy or polyurethane systems or a corresponding primary amine forpolyimide systems, with an acetic acid catalyst and Zr hydrolysis ratestabilizer for aqueous formulations.

The sol usually is filled with pigments to provide desired, color,gloss, reflectivity, electrical conductivity, emissivity, or the like.The sol-gel film is usually applied by spraying or drenching the metalin or with the sol without rinsing. For metal surfaces, the sol-gel filmproduces a gradient changing from the characteristics of metal to thoseof organic resins. Good adhesion results from clean, active metalsurfaces with sol coatings that contain the organometallic couplingagents in the proper orientation. After application, the sol coating isdried at ambient temperature or, more commonly, heated to a temperaturebetween ambient and 250° F. to complete the sol-gel film formation.

Ideally, covalent bonding occurs between the metal surface and azirconium compound in the sol. Successful bonding requires a clean andchemically active metal surface. The strength and durability of the solcoating depends upon chemical and micro-mechanical interactions at thesurface involving, for example, the metal's porosity and microstructureand on the susceptibility of the sol coating to rehydrate. The methodsused to prepare the surface for the sol-gel coating are part of thecoating sequence of the present invention.

The sol-gel is normally applied in several coats to form the finalcoating. It may be beneficial to vary the chemistry of the sol-gelslightly between coats. For example, the initial coats might use TEOS(tetra-ethyl-orthosilicate) as the organosilane to produce a moreceramic-like layer while the later coats might use an aminosilane withan active functional group to enhance bonding with overcoats, adhesives,or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the typical steps in the surface treatmentprocess of the present invention.

FIG. 2 is a graph showing wedge crack extension for a sol-coatedtitanium alloy of the present invention compared with a chromic acidanodize standard.

FIG. 3 is a graph showing wedge crack extension for a sol-coatedaluminum alloy of the present invention compared with a phosphoric acidanodized standard.

FIG. 4 is a chart showing lap shear ultimate stress test results forseveral coupons of titanium-6Al-4V alloy adhered with Cytec FM-5polyimide adhesive.

FIG. 5 is a chart showing floating roller peel resistance test resultsfor sol-coated 2024 and 7075 aluminum alloys compared with phosphoricacid anodize standards.

FIG. 6 is a graph showing cumulative crack growth of alcohol-based andwater-based sol coatings on a titanium alloy as a function of theduration of exposure to hot/wet conditions.

FIG. 7 is a graph showing the effect of surface cleaning andpretreatment by plotting cumulative crack growth against the duration ofexposure to hot/wet conditions for samples with differing surfacetreatments.

FIG. 8 is a graph showing the effect of drying time at 230° F. on thetime to failure of wedge crack extension test samples.

FIG. 9 is a graph showing the effect of spraying versus dipping(immersing) to apply the coating, plotting cumulative crack growthagainst the duration of exposure to hot/wet conditions.

FIG. 10 is a graph showing cumulative crack growth as a function ofextended exposure to hot/wet conditions comparing sol-coated metals withchromic acid anodized standards.

FIG. 11 is an isometric view of a typical hybrid laminate.

FIG. 12 is an isometric view of a sandwich panel having hybrid laminateskins and a honeycomb core as typically used in an aerospace skin panel.

FIG. 13 is a sectional view showing the typical layers in a sol coatedmetal product, here illustrated as a lap joint having an adhesive bond.

FIG. 14 is a schematic sectional view of the sol coating.

FIG. 15 is another schematic sectional view of the sol coating for paintadhesion showing the interfaces at the metal and resin interfaces and-the-peptizing (i.e. crosslinking between the metals) within the solcoating between the Zr and Si.

DETAILED DESCRIPTION OF THE INVENTION

First, we will discuss some generally applicable aspects of the sol andof the sol coating. Then, we will discuss the pigmented sol that is thefocus of the present invention.

1. The Sol Coating

Sol coating of metals achieves resin-to-substrate bonding via chemicallinkages (covalent bonds, hydrogen bonds, or van der Waals forces) whileminimizing environmental impacts otherwise caused by the traditional useof highly diluted hazardous metals. A preferred sol for making the solcoating (also called a sol-gel film) on a metal substrate includes anorganozirconium compound (such as tetra-n-propoxyzirconium) to bondcovalently to the metal through Zr and an organosilane (such as3-glycidoxypropyltrimethoxysilane) to bond covalently to the organicprimer, adhesive, or resin (with an acetic acid catalyst in water-basedformulations as a catalyst for the silane and as a hydrolysis ratestabilizer for the zirconate).

In a successful surface treatment, the typical failure mode foradhesively bonded specimens in a hot/wet environment is cohesive failurein the organic adhesive layer. In such cases, the sol-gel film isstronger than the bulk adhesive, so the adhesive bond is as strong aspossible.

We use sol-gel chemistry to develop binder coatings about 20-500 nmthick that produce a gradient from the metallic surface through a hybridorganometallic sol-gel film to the adhesive. Polishing the surface mayimprove our control of the coating thickness. If the film is too thick,it becomes glassy. Bond strength and durability in our preferred solcoating is increased by including organosilanes and organozirconiumcompounds. The organosilanes covalently bond to or otherwise associatewith the organic adhesive resin or primer. Ideally, covalent bondingalso occurs at the interface between the sol-gel and metal surface.Mechanical interactions may also play a role depending on the design(i.e., porosity, microstructure) of the substrate or the sol coating.Durability of the sol-gel film in humid conditions depends on whetherthe film rehydrates.

The term “sol-gel,” a contraction of solution-gelation, refers to aseries of reactions where a soluble metal species (typically a metalalkoxide or metal salt) hydrolyzes to form a metal hydroxide. Thesoluble metal species usually contain organic ligands tailored tocorrespond with the resin in the bonded structure. The metal hydroxidescondense (peptize) in solution to form a hybrid organic/inorganicpolymer. Depending on reaction conditions, the metal polymers maycondense to colloidal particles or they may grow to form a network gel.The ratio of organics to inorganics in the polymer matrix is controlledto maximize performance for a particular application.

Many metals are known to undergo sol-gel reactions. Silicon and aluminumsol-gel systems have been studied extensively. Representative sol-gelhydrolysis and condensation reactions, using silicon as an example, areshown in equations (1) and (2).Si(OEt)₄+2H₂O

Si(OH)₄+4 EtOH   hydrolysis (1)Si(OH)₄

SiO₂+2 H₂O   condensation (2)wherein Et is ethyl (CH₃CH₂—). The hydrolysis and condensation reactionscan be complete, resulting in complete conversion into the metal oxideor a hydrous metal hydroxide. They can also be partial, leaving more ofthe alkoxide functionalities in the finished gel. Depending upon thereaction conditions, reactions (1) and (2) can produce discrete oxideparticulates, as demonstrated in the synthesis of nanoscale particles,or they can form a network gel, which can be exploited in filmformation. The solubility of the resulting gel in a solvent will dependupon the size of the particles and degree of network formation.

Surface preparation is important, if not critical, to produce strong,durable bonds. We prefer a clean and chemically active metal surface tobond a sol-gel film from the sol by spraying, immersing, or drenching.Cleaning is a key factor toward obtaining good adhesion. If the surfaceis dirty, bonding is blocked by the dirt or occurs between the sol andthe dirt rather than between the sol and the surface. Obtaining achemically active surface is not trivial. Titanium produces a passiveoxide surface. A bare, pure titanium surface will immediately oxidize inair or dry oxygen to form a barrier titanium oxide film which has athickness of 2-4 nm (20-40 Å). Titanium surface oxides do not hydrolyzeas readily as aluminum surface oxides to form active metal hydroxides.Water will, however, chemisorb onto the surface of the titanium oxide.Aluminum oxidizes as quickly, or more quickly in air.

HNO₃—HF etching of titanium alloys removes TiO₂ alpha case, but createsa smooth surface which is difficult to bond to. Traditional alkalineetches like TURCO 5578 or OAKITE 160, produce a roughened surface bettersuited for adhesive bonding, but produce a tenacious smut layer. Thesmut causes adhesion to be reduced dramatically. Extended soaking in hotHNO₃ after the alkaline etch still leaves some smut. In our preferredprocess, we clean and rinse the surface, etch with HNO₃—HF, rinse again,and alkaline etch. Again after another rinse, we desmut the surface withBOECLENE once or in multiple stages to produce a clean and activesurface best suited for adhesive bonding through the sol coating of thepresent invention.

Our model of the formation of a sol-gel film on titanium involves Lewisacid/base interaction of a hydrolyzed zirconium alkoxide, anorganosilane, or both in the sol with the titanium oxide surface. Thisinteraction is possibly assisted by chemisorbed water to cause theformation of a coupled Zr—O—Ti or Si—O—Ti linkage and a new Ti—OH bondon the surface. A similar reaction occurs on aluminum. The ability ofthe metal alkoxides to covalently bond with the metal surface mostlikely requires more energy in the case of titanium than aluminum.Complete coupling and formation of covalent bonds with titanium alloysmay not occur until the part reaches higher temperatures, such as theyusually experience during adhesive curing.

Sol-gel chemistry is quite versatile. Reaction conditions (for example,concentration of reagents and catalyst type) control the relative ratesof the hydrolysis and condensation reactions. Sol-gel solutions can beprepared which readily form thin films or which condense to finecolloidal particles. Starting materials and reaction conditions canproduce films with morphology similar to surface coatings achieved withanodize and etch processes. Density, porosity, and microstructure can betailored by controlling the chemistry of the sol.

Sol-gel condensation reactions are affected by the acid-base characterof the metal/oxide surface. The isoelectronic point (IEP, a measure ofsurface pH) for titanium is more acidic (IEP=6.0) than an aluminumsurface (IEP=9.2), which changes the surface chemistry of the metal withthe sol.

2. Screening Studies on Sol Coatings

We conducted screening studies to define the sol formulation on testpanels-of titanium-6Al-4V (Ti-6-4) alloy sized 6″×6″×0.50″ initiallyprepared by degreasing the surface with an aqueous detergent (11, FIG.1). The panels were then either grit blasted with #180 grit alumina (13)followed by a final aqueous detergent cleaning to minimize the presenceof loosely adhered grit or acid etched in a HNO₃—HF immersion tank (notshown in FIG. 1). Our preferred sol for these tests consisted of adilute aqueous mixture of 3-glycidoxypropyltrimethoxysilane (GTMS) andtetra-n-propoxyzirconium (TPOZ) with an acetic acid catalyst. Typically,the panels were dip-coated with a 10 minute immersion time (15), heldunder ambient conditions for 15 to 30 minutes (17), and dried in a 230°F. oven for 15-30 minutes (19). With the sol coating complete thespecimens were ready for accepting primer (21) and then an epoxyadhesive (23). We also tested corresponding formulations using alcoholas the carrier or solvent. These epoxy sols typically have a pH around4-5.

Our test specimens were primed with BMS 5-89 chromated adhesive primer(American Cyanamid BR127). Two sol coated panels were then bondedtogether to form an adhesive lap joint in an autoclave using BMS 5-101Type II Grade (Dexter-Hysol EA 9628) 250° F. cure epoxy adhesive.

Screening level testing used the ASTM D 3762 Wedge Test with exposure at140° F. and greater than 95% relative humidity to test the bondstrength. The bonded panels were cut into five 1″×6″ strip specimens andwedges were driven into the bondline. Progress of the crack along thebondline was measured after the initial driving of the wedge, and afterexposure to 140° F. and greater than 95% relative humidity for one hour,24 hours, one week, and longer. Samples were monitored in the humiditychamber for over 2500 hours total exposure time. Typical test resultscompared with conventional chromic acid anodizing (CAA) are shown inFIG. 2. Test data for comparable aluminum specimens with 7075 or 2024aluminum alloys are shown in FIG. 3. Here, the standard surfacetreatment for comparison was phosphoric acid anodizing (PAA).

FIG. 4 reports test results of the ultimate strength of Ti-6-4/FM-5polyimide adhesive lap joints. FIG. 5 reports the average roller peelresistance of aluminum/epoxy specimens similar to those made for thecrack growth tests reported in FIG. 3.

Additionally, lap shear testing showed good wedge crack screeningcharacteristics. Finger panels were primed and lap bonded with the 5-101adhesive. Measurements were taken at −65° F., room temperature, and 165°F.

A water-based system alleviates many of the flammability, safety,toxicity, and environmental concerns associated with the process whenthe sol is alcohol-based. We chose a glycidoxysilane (an epoxy) becauseof its stability in solution and its ability to crosslink with common,aerospace epoxy, urethane, or cyanate ester adhesives. The silane isacid-base neutral (pH=7.0) so its presence in the sol mixture does notincrease the relative hydrolysis and condensation rates of thealkoxides. Sols including the organosilanes are relatively easy toprepare and to apply with reproducible results.

The choice of the organosilane coupling agent was a significant factorin improving hot/wet stability of the BMS epoxy bonding system. The GTMShas an active epoxy group which can react with the bond primer. GTMS didnot form strong Lewis acid-base interactions with the hydrated titaniumoxide substrate. The titanium oxide surface was more accessible to thezirconium organometallic when GTMS was used, allowing the desiredstratification of the sol-gel film in essentially a monolayer with theepoxy groups of the silane coupling agents oriented toward the primer.We believe this orientation allowed strong covalent bonding to developbetween the titanium substrate and zirconia and silica (i.e. M-O-Mbonds), as well as maximizing bonding between the epoxy moiety of theGTMS to the epoxy adhesive. The ideal concentration will depend upon themode of application. A higher concentration may be preferred for drenchor spray applications.

For polyimides (including bismaleimides), we usually replace GTMS withan aminoalkylsilane or an aminoarylsilane to match the silane couplingchemistry with the resin system. In these sols which operate best at apH 8-9, we add ammonium hydroxide in small amounts to adjust the pH andto disrupt the Zr-acetate complex that otherwise forms.

Physical size of the silane coupling agent also has an effect onadhesion. Aluminum studies revealed that both the initial adhesion andhydrolytic stability decreased whenepoxycyclohexylpropyltrimethoxysilane was used instead of GTMS as thecoupling agent. We attribute the difference in performance to adifference in size of the organic functionality. This size effect ismost likely the result of physical interference of both hydrolysis andcondensation reactions by the bulky alkyl group attached directly to thesilicon. Hydrolysis was incomplete and the silicon hydroxide could noteffectively condense with the aluminum surface. These results suggestthat the most effective coupling agents for a spray or drench coatingapplication will be smaller so as not to sterically hinder hydrolysisand condensation reactions.

The concentrations of the reactants in the sol were generally determinedas volume percentages. In the screening tests, a 2 vol % of GTMS and 1vol % of TPOZ was used. This concentration corresponds to a molar ratioof silicon to zirconium of 3.7:1. More organosilane is generally used inpolyimide formulations than for the epoxy formulations that use GTMS.Related studies suggest that a slightly higher concentration ofreactants, namely a total of (Si+Zr)=4.4 volume % may yield betterresults, so the ratio of GTMS to TPOZ might need further adjustment toobtain the optimal performance (strongest surface adherence). We believethat a higher adhesion will occur with a mixed (Zr+Si) sol because ofthe more chemically active Zr. The sol might also include cerium,yttrium, titanium, or lanthanum organometallics, such as yttrium acetatetrihydrate or other hydrates, yttrium 2-ethylhexanoate,i-proproxyyttrium, methoxyethoxyyttrium, yttrium nitrate, cerium acetatehydrate, cerium acetylacetonate hydrate, cerium 2-ethylhexanolate,i-propoxycerium, cerium stearate, cerium nitrate, lanthanum nitratehexahydrate, lanthanum acetate hydrate, or lanthanum acetylacetonate, oran aluminum alkoxide, together with the Zr or in its place.

The organozirconium compound reduces or minimizes the diffusion ofoxygen to the surface and to stabilize the metal-resin interface. As avariation to the sol coating process, a stabilizer might be applied tothe surface to form a barrier film prior to applying the hybridorganometallic sol to form the sol-gel film.

Preliminary screening tests were also conducted with and withoutconductive pigments on various substrates. For the purpose of thisinitial study, a 10% pigment loading level (with respect to sol-gelbinder weight) was used to examine coating uniformity and sprayability.In these tests, we used aluminum and nickel metallic fillers. While wedo not expect these fillers to be in the optimized formulations, theywere a low cost way of testing compositional changes and loading levelsfor the conductive systems.

Table 1 describes sample configuration and initial results from thesescreening studies. Within a series of specimens, four differentsubstrates were tested (aluminum 7075-T6, cyanate ester-carbon fibercomposite, mylar, and glass) at three coating thicknesses. Samplesseries #2 consisted of the unpigmented silicon-zirconium sol-gel binderhaving GTMS and TPOZ at a concentration of 10 vol %. This series wascarried out to get control information about the coating system. Thepigmented sols used 10 vol % GTMS/TPOZ sol as the vehicle. Sample series#3 added an aluminum leafing pigment to the GTMS/TPOZ sol while series#4 and #6 used an aluminum non-leafing pigment at 10% and 20% by volume,respectively. Aluminum proved to be a poor pigment with which to achievea conductive coating. Sample series #5 and #12 added a nickel flakepigment at different concentrations. Sample series #8, #10, #13, #14,#17, and #18 used carbon black at several concentrations in theGTMS/TPOZ sol. While conductivity could be obtained at higherconcentrations, the adhesion of the sol-gel was reduced. Sample series#15, #16, #19, and #20 used different concentrations of tin oxide, andachieved conductive coatings at concentrations of 30 vol % pigment.Sample series #21, #22, and #26 added different concentrations ofsilvered spheres, but failed to provide acceptable, conductive, sol-gelcoatings. Sample series #24 added indium tin oxide, but obtained noresponse at a concentration of 20 vol %. Finally, sample series #23 and#27 added different mixtures of nickel flake and carbon black to achieveconductive sol-gel coatings on glass. TABLE 1 Conductive PigmentScreening Tests with Sol-Gel Coating (GTMS/TPOZ 10 vol %) CoatingSurface Sample Pigment Thickness Resistivity # Conductive filler Vol %Substrate (mils) (ohms/sq) Comments 2-1 none 0 Al 7075 T6 0 did not wet2-2 none 0 C fib/cyan est out on Mylar 2-3 none 0 Mylar 0.1 no response2-4 none 0 glass 0 no response 2-5 none 0 Al 7075 T6 0.1 2-6 none 0 Cfib/cyan est 2-7 none 0 Mylar 0.1 no response 2-8 none 0 glass 0.2 noresponse 2-9 none 0 Al 7075 T6 0.2 2-10 none 0 C fib/cyan est 2-11 none0 Mylar 0.2 no response 2-12 none 0 glass 0.2 no response 3-1 Al 13 umleaf 10 Al 7075 T6 0.4 Al leaf did 3-2 Al 13 um leaf 10 C fib/cyan estnot disperse 3-3 Al 13 um leaf 10 Mylar 0.3 no response 3-4 Al 13 umleaf 10 glass 0.2 no response 3-5 Al 13 um leaf 10 Al 7075 T6 0.4 3-6 Al13 um leaf 10 C fib/cyan est 3-7 Al 13 um leaf 10 Mylar 0.5 no response3-8 Al 13 um leaf 10 glass 0.4 no response 3-9 Al 13 um leaf 10 Al 7075T6 0.6 3-10 Al 13 um leaf 10 C fib/cyan est 3-11 Al 13 um leaf 10 Mylar0.4 no response 3-12 Al 13 um leaf 10 glass 0.4 no response 4-1 Al 17 umnleaf 10 Al 7075 T6 0.4 Coated nicely 4-2 Al 17 um nleaf 10 C fib/cyanest 4-3 Al 17 um nleaf 10 Mylar 0.3 no response 4-4 Al 17 um nleaf 10glass 0.2 no response 4-5 Al 17 um nleaf 10 Al 7075 T6 0.6 4-6 Al 17 umnleaf 10 C fib/cyan est 4-7 Al 17 um nleaf 10 Mylar 0.5 no response 4-8Al 17 um nleaf 10 glass 0.5 no response 4-9 Al 17 um nleaf 10 Al 7075 T60.6 4-10 Al 17 um nleaf 10 C fib/cyan est 4-11 Al 17 um nleaf 10 Mylar0.6 no response 4-12 Al 17 um nleaf 10 glass 0.6 no response 5-1 Niflake 10 Al 7075 T6 0.3 Ni settled out 5-2 Ni flake 10 C fib/cyan estmust agitate 5-3 Ni flake 10 Mylar 0.4 no response briskly 5-4 Ni flake10 glass 0.2 no response 5-5 Ni flake 10 Al 7075 T6 0.5 5-6 Ni flake 10C fib/cyan est 5-7 Ni flake 10 Mylar 0.5 no response 5-8 Ni flake 10glass 0.6 .51 E7 5-9 Ni flake 10 Al 7075 T6 0.4 5-10 Ni flake 10 Cfib/cyan est 5-11 Ni flake 10 Mylar 0.5 .33 E5 5-12 Ni flake 10 glass0.6 .24 E5 6-1 Al 17 um nleaf 20 Al 7075 T6 0.5 sprayed well 6-2 Al 17um nleaf 20 C fib/cyan est 0.4 opaque 6-3 Al 17 um nleaf 20 glass 0.6 noresponse 6-4 Al 17 um nleaf 20 Al 7075 T6 0.7 6-5 Al 17 um nleaf 20 Cfib/cyan est 0.6 6-6 Al 17 um nleaf 20 glass 0.8 no response 8-1 Carbonblack 4 Al 7075 T6 0.5 splotchy 8-2 Carbon black 4 C fib/cyan est 0.4appearance, 8-3 Carbon black 4 glass 0.5 no response poor pigment 8-4Carbon black 4 Al 7075 T6 0.7 dispersion 8-5 Carbon black 4 C fib/cyanest 0.6 8-6 Carbon black 4 glass 0.7 no response 10-1 Carbon black 5 Al7075 T6 0.3 surfactant 10-2 Carbon black 5 C fib/cyan est 0.1 added,10-3 Carbon black 5 glass 0.3 .68 E6 did not 10-4 Carbon black 5 Al 7075T6 0.5 spray 10-5 Carbon black 5 C fib/cyan est 0.3 well 10-6 Carbonblack 5 glass 0.3 .94 E5 12-1 Ni flake 20 Al 7075 T6 0.8 coating isrough 12-2 Ni flake 20 C fib/cyan est 0.6 12-3 Ni flake 20 glass 0.6 .98E2 12-4 Ni flake 20 Al 7075 T6 1.1 12-5 Ni flake 20 C fib/cyan est 0.512-6 Ni flake 20 glass 1.1 .54 E2 13-1 Carbon black 5 Al 7075 T6 0.3different 13-2 Carbon black 5 C fib/cyan est 0.2 surfactant, 13-3 Carbonblack 5 glass 0.3 no response sprayed OK 13-4 Carbon black 5 Al 7075 T60.3 13-5 Carbon black 5 C fib/cyan est 0.3 13-6 Carbon black 5 glass 0.1.47 E6 14-1 Carbon black 20 Al 7075 T6 na milled longer 14-2 Carbonblack 20 C fib/cyan est na with surfactant, 14-3 Carbon black 20 glassna .21 E4 sprayed OK 14-4 Carbon black 20 Al 7075 T6 0.9 but cracks 14-5Carbon black 20 C fib/cyan est 0.9 appeared 14-6 Carbon black 20 glass0.7 .20 E4 15-1 Tin oxide - 1 10 Al 7075 T6 0.3 Coating had fish 15-2Tin oxide - 1 10 C fib/cyan est 0.5 eyes and didn't 15-3 Tin oxide - 110 glass 0.3 no response cover well 15-4 Tin oxide - 1 10 Al 7075 T6 0.515-5 Tin oxide - 1 10 C fib/cyan est 0.4 15-6 Tin oxide - 1 10 glass 0.7no response 16-1 Tin oxide - 1 20 Al 7075 T6 0.4 not very opaque, 16-2Tin oxide - 1 20 C fib/cyan est 0.6 poor coverage 16-3 Tin oxide - 1 20glass 0.4 no response 16-4 Tin oxide - 1 20 Al 7075 T6 0.5 16-5 Tinoxide - 1 20 C fib/cyan est 0.4 16-6 Tin oxide - 1 20 glass 0.5 noresponse 17-1 Carbon black 15 Al 7075 T6 0.3 sprayed well 17-2 Carbonblack 15 C fib/cyan est 0.3 initially but after 17-3 Carbon black 15glass 0.2 .12 E4 6 passes cracks 17-4 Carbon black 15 Al 7075 T6 0.5formed 17-5 Carbon black 15 C fib/cyan est 0.5 17-6 Carbon black 15glass 0.8 .24 E4 18-1 Carbon black 25 Al 7075 T6 0.4 cracked after 418-2 Carbon black 25 C fib/cyan est 0.3 passes, 18-3 Carbon black 25glass 0.3 .15 E4 poor adhesion 18-4 Carbon black 25 Al 7075 T6 1.3 18-5Carbon black 25 C fib/cyan est 1 18-6 Carbon black 25 glass 1.4 .40 E419-1 Tin oxide - 1 30 Al 7075 T6 0.6 sprayed well 19-2 Tin oxide - 1 30C fib/cyan est 0.2 19-3 Tin oxide - 1 30 glass 0.4 .18 E7 19-4 Tinoxide - 1 30 Al 7075 T6 0.7 19-5 Tin oxide - 1 30 C fib/cyan est 0.819-6 Tin oxide - 1 30 glass 0.6 .37 E6 20-1 Tin oxide - 2 30 Al 7075 T60.3 sprayed well 20-2 Tin oxide - 2 30 C fib/cyan est 0.3 20-3 Tinoxide - 2 30 glass 0.3 .36 E6 20-4 Tin oxide - 2 30 Al 7075 T6 0.5 20-5Tin oxide - 2 30 C fib/cyan est 0.5 20-6 Tin oxide - 2 30 glass 0.4 .17E6 21-1 Silvered sphere 10 Al 7075 T6 1.5 very low coverage 21-2Silvered sphere 10 C fib/cyan est 1.3 21-3 Silvered sphere 10 glass 1.7no response 21-4 Silvered sphere 10 Al 7075 T6 1.8 21-5 Silvered sphere10 C fib/cyan est 1.2 21-6 Silvered sphere 10 glass 1.9 no response 22-1Silvered sphere 20 Al 7075 T6 1.8 low coverage 22-2 Silvered sphere 20 Cfib/cyan est 1.6 nearly transparent 22-3 Silvered sphere 20 glass 1.6 noresponse after 8 passes 22-4 Silvered sphere 20 Al 7075 T6 2 22-5Silvered sphere 20 C fib/cyan est 1.7 22-6 Silvered sphere 20 glass 1.9no response 23-1 Ni/Carbon bl 15/5 Al 7075 T6 0.7 sprayed well 23-2Ni/Carbon bl 15/5 C fib/cyan est 0.5 23-3 Ni/Carbon bl 15/5 glass 0.5.15 E3 23-4 Ni/Carbon bl 15/5 Al 7075 T6 1.2 23-5 Ni/Carbon bl 15/5 Cfib/cyan est 1.2 23-6 Ni/Carbon bl 15/5 glass 1.1 .26 E2 24-1 In/Snoxide 20 Al 7075 T6 0.7 not very uniform 24-2 In/Sn oxide 20 C fib/cyanest 0.4 settled while 24-3 In/Sn oxide 20 glass 0.5 no response spraying24-4 In/Sn oxide 20 Al 7075 T6 0.8 24-5 In/Sn oxide 20 C fib/cyan est0.6 24-6 In/Sn oxide 20 glass 0.7 no response 25-1 Tin oxide - 1 40 Al7075 T6 0.6 coating very 25-2 Tin oxide - 1 40 C fib/cyan est 0.4 opaqueand 25-3 Tin oxide - 1 40 glass .17 E7 uniform 25-4 Tin oxide - 1 40 Al7075 T6 1.1 25-5 Tin oxide - 1 40 C fib/cyan est 1.2 25-6 Tin oxide - 140 glass .61 E6 26-1 Silvered sphere 30 Al 7075 T6 1.8 did not spray26-2 Silvered sphere 30 C fib/cyan est 1.6 well, 26-3 Silvered sphere 30glass no response non-uniform lay 26-4 Silvered sphere 30 Al 7075 T6 2.1down of spheres, 26-5 Silvered sphere 30 C fib/cyan est 1.9 tend tomigrate 26-6 Silvered sphere 30 glass no response 27-1 Ni/Carbon bl10/2.5 Al 7075 T6 0.6 sprayed well 27-2 Ni/Carbon bl 10/2.5 C fib/cyanest 0.4 27-3 Ni/Carbon bl 10/2.5 glass 1.47 E3 27-4 Ni/Carbon bl 10/2.5Al 7075 T6 0.9 27-5 Ni/Carbon bl 10/2.5 C fib/cyan est 1.1 27-6Ni/Carbon bl 10/2.5 glass 1.09 E2

Surface conductivity measurements were conducted using a four-pointprobe under ambient conditions. Most of the specimens in this seriesshowed no electrical response down to the detection limits(approximately 1×10⁸ ohm/sq) of the probe. The sol-gel binder alone(series #2) was not expected to be conductive. Pigmentation at a 10%loading level was not high enough to produce conductivity in thesealuminum specimens. The 0.5 mil-thick sol-gel coating doped with nickelflake was slightly conductive, showing values of approximately 3×10⁴ohms/sq.

Mechanical evaluation of the coatings in Table 1 consisted of tapeadhesion tests and some impact testing. The adhesion testing shows theoriginal binder (series #2) has excellent adhesion on both metal andcomposite substrates. The adhesion test results varied with addition offillers, with some loss of adhesion occurring in certain pigment loadedspecimens. At this point, this loss of adhesion is not of great concern.No special care or surface treatments on the substrates were used inthese screening studies. The substrate surfaces were simply degreasedand slightly roughed using a Scotchbrite pad. After a coating withacceptable conductivity has been formulated, we will go back andoptimize the adhesion aspects of the formulation and interface.

The sol can be applied in several coats and may have a gradient fromceramic character to more organic character in depth from adjacent themetal surface to the interface with a paint, resin composite, oradhesive. Ceramic character can be enhanced by minimizing the organicside chains (ligands) on the Si in the sol. For example, we might usetetra-ethyl-orthosilicate (TEOS) as the organosilane for the first coatof sol. Then, we may apply a layer having equal amounts of TEOS andGTMS. Dissolve the TEOS in alcohol to hydrolyze it before adding themixture to water. The last coat in this example might use only GTMS asthe organosilane. We believe that a more ceramic character adjacent themetal might create a more effective barrier to block water or othercorrosive agents. Similar adjustments might be made with theorganozirconium to produce stronger ceramic character. Of course,additional or fewer layers might be applied. Commonly we apply 3-6layers or coats of the sol even if we do not tailor the sol-gel byadjusting the organometallic ingredients in the sol. In all cases,however, we maintain the ratio of the Zr:Si in each layer.

Alcohol-based sols allow us to precisely control the amount ofhydrolysis. Optimization of the water-based system, however, actuallyyielded better hot/wet durability results than the alcohol-based system,as demonstrated by comparing similar alcohol and water-based coatings(FIG. 6). These results, however, may vary if the alcohol-based solincludes hydrolysis control for the zirconium.

Aging of the sol which we call the “induction time” is another importantfactor in using our sols. Complete hydrolysis and condensation of theorganometallic in the sol-gel film is important to develop ahydrolytically stable metal oxide film in the metal. The presence ofhydrolyzable alkoxides in the sol-gel film will have two adverseeffects. First, every residual alkoxide represents an incompletecondensation site between the metal and the coupling agents. Incompletecondensation, therefore, decreases the ultimate bond strength of thesol-gel film. Second, in a humid environment, these alkoxide residuescan hydrolyze. The structural changes accompanying hydrolysis causestress in the sol-gel film which, we believe, promotes failure to occurwithin the sol-gel film or at one of the interfaces (metal/film orfilm/primer or adhesive).

Aging is a function of the rates of the hydrolysis reaction of thezirconium alkoxides and the organosilane. Tetra-n-propoxyzirconiumreacts more rapidly with water or other active hydrogens than thesilane. The zirconate hydrolyzes rapidly using ambient moisture andcondenses with itself or with absorbed water on the titanium surface. Ifnot properly controlled, this zirconate hydrolysis self-condensationreaction can produce insoluble zirconium oxyhydroxides which willprecipitate and become nonreactive.

If, however, the sol is applied too short a time after being made, theorganosilane may not be fully hydrolyzed. As the sol ages, thehydrolyzed silicon and zirconium components may condense amongthemselves, forming oligomers and networks. These networks willeventually become visible to the naked eye and become insoluble. Theideal solution age is at the point that the zirconium and silicon arehydrolyzed sufficiently that zirconium and silicon react with the metalsurface. At this point, generally some metal polymers and networks haveformed in the sol and they will give the sol-gel film some structure.

We made the zirconium and silicon components hydrolyze on a similar timescale by mixing the zirconium alkoxide with glacial acetic acid tostabilize the fast reacting four-coordinate zirconate center and toenable a water-based system. This mixing effectively changed thegeometric and electronic nature of the zirconium component. Otherorganic acids, like citric acid, can be substituted for the acetic acid.We can also use glycols, ethoxyethanol, H₂N—CH₂—CH₂—OH, or the like.

The relative rates of the hydrolysis and condensation reactions involvedin the sol coating process are controlled by the type of catalyst(either acid or base), the concentrations of the reagents in thereactions, the metal alkoxide selected, and the water available forhydrolysis. An acidic catalyst promotes the hydrolysis reaction overcondensation while a basic catalyst does the opposite. We examined theeffects of various acidic catalysts, such as acetic acid and nitricacid, and basic catalysts, such as ammonium hydroxide and triethylamine.For these formulations, the basic catalysts promoted the condensationreactions too vigorously, which shortened the pot-life of the solution.Colloidal zirconate-silicate particles precipitated too soon after thesol was mixed. The nitric acid was effective as a catalyst, but did notstabilize the zirconate via a coordinating ligand like the acetate ionin acetic acid, so aging of the sol produced differing, unpredictableresults. Thus, acetic acid was chosen as the preferred catalyst. We makethe sols dilute to control the self-condensation reactions, therebyextending the pot life. Still, the sols must be used soon after they areprepared.

Acetic acid functions as a catalyst for the hydrolysis of theorganosilane and as a hydrolysis rate stabilizer for the zirconiumcomplex. The acetic acid helps to make both ingredients ready forbonding at comparable rates. In general throughout our screening tests,we added 0.13 moles of glacial acetic acid to 0.091 moles of theorganozirconium before combining the organosilane with theorganozirconium. We have not optimized the amount of glacial aceticacid, however, in our initial screening tests.

In our tests, we cleaned the metal surface using abrasive blasting oracid etching with HNO₃—HF in both liquid and paste form. Since the solreacts directly with chemical moieties on the substrate surface,adhesion is sensitive to surface precleaning. Residues or smut resultingfrom the cleaning processes can drastically effect the adhesive bondperformance, because residues and smut are relatively loosely adhered tothe surface.

FIG. 7 shows the wedge crack test results for Ti-6Al-4V panels given avariety of surface pretreatments and then coated with the same sol. Ourresults indicate that degreasing with an aqueous detergent with a gentlescrubbing action or agitation was sufficient for removing most soils andgrease from the metal surface. Subsequent grit blasting was generallybetter than acid etching for pretreatment of the surface. We believethat grit blasting enhanced mechanical interaction by producing amacrorough surface. The grit blasted surface may hold the sol on thesurface longer during the ambient temperature flash, allowing a longerreaction time between the sol and surface, but the time difference israther short. Prehydrolysis of the surface using steamy or hot water mayactivate the metal by populating the surface with chemisorbed water. Thewater on the surface can react with the activated surface to producesurface hydroxyls which are available for condensation with the sol.Surface hydroxyls are especially important for titanium alloys.

The two best performing sets of samples in our pigment-free sol wereboth grit blasted with #180 alumina and had the roughest surfaces. Ofthese two grit blasted samples, the set which was cleaned with Alconoxdetergent following grit blasting had better performance than one wipedonly with methyl ethyl ketone (MEK). Solvent wiping of the roughsurfaces of grit blasted panels with lint free cloth frequently leftsmall shreds or fiber residues on the substrates. Cloth residuesincreased with higher pressure exerted on the wiping cloth. Test resultsfor panels etched for one minute in HNO₃—HF did not perform as well asthe grit blasted panels. The poorest wedge crack test performance wasobtained from panels abraded by hand with #80 and #150 grit siliconcarbide sandpaper. The sandpaper produced a relatively non-uniformsurface that typically was contaminated with silicon carbide. Detergentwashing did not remove the contamination. Sanding does not produce thesame mechanical surface as grit blasting.

A paste etch process was considered as an alternative to the HNO₃—HFacid etch bath for field repair. The paste mixed the nitric andhydrofluoric acid in an emulsifier. It was applied with a brush on thesurface of the titanium panels. Four 6″×5″ titanium panels were solventwiped and prepared for the process. Hydrogen bubbles were produced onthe surface of the metal during the process by the reaction of the acidand the metal surface. These bubbles became encapsulated in the paste. Acontinuous brushing motion over the surface of the panel was necessaryto keep the etchant in contact with the titanium. Without brushing, theetching was uneven.

TURCO 5578 alkaline etch produces a mat finish, similar to an anodize,resulting from the formation of a microrough surface. This pretreatmentshows superior hot/wet durability.

The use of conventional dry grit blasting as a surface preparationpretreatment prior to sol coating has both advantages and disadvantagesdepending on the type of metal surface to be treated and the environmentin which the process is being carried out. Grit blasting produced thehighest strength and most durable adhesive bonds of the tested surfacetreatments over the course of this program. Grit blasting should workwell in practice on thicker panels or parts requiring limited amounts ofblasting. Care must be taken not to warp the panel as the result ofstresses introduced during the blasting.

Grit blasting cannot be used on titanium foil or-honeycomb core withoutserious risk of damage to the substrate or on fatigue critical parts.Blasting produces holes in the metal in these cases. Complex parts mightbe difficult to access to produce a uniform finish. Although theequipment and materials required for grit blasting are not exotic, theymay not be available at sites where sol coatings could otherwise easilybe applied. We do not use it when applying pigmented sols.

In other tests, we blasted titanium panels with different grit sizes ofalumina #46 grit, #180 grit, and a very fine polishing alumina with anaverage size of 50 micrometers. All of the grit sizes evenly abraded thesurface and yielded a uniform matte finish. Surface roughness wasmeasured using a surface profilometer, using a half inch traverse and a0.03 cutoff. The average roughness (R_(a)) was 144 microinches for the#46 grit panel, 30 for the #180 grit panel, and 22 for the panel blastedwith the fine grit. Surface contaminants, like heavy greases or oils,were not easily removed during grit blasting using the fine aluminapowders. The grit was simply imbedded into the contaminant and lost allvelocity. The finest alumina grit was too friable and broke down quicklyduring the blasting process. After a certain time period, the very fineparticles were no longer effective at abrading the surface. The dust washard to contain within the sandblasting apparatus.

We used scanning Auger microscopy to analyze the panels after blasting.The effective oxide thickness of the non-blasted area was measured atapproximately 200 Å, while the total structured surface was about 2000 Åfor that of the blasted area. Bright particles were observed imbeddedinto the surface and were verified to contain primarily aluminum andoxygen, most likely as Al₂O₃. Acid etching removed the embedded aluminaparticles from the titanium surface only after the titanium had beenetched away and removed from around the particles. Unfortunately,post-blasting etching also removed the roughened texture of the surface.We do not know the role alumina particles play, if any, in adhesion tothe grit blasted surface.

No single surface pretreatment appears to provide optimum results overthe full range of substrates. Various combinations of acid etch andalkaline etch treatments apparently work well on certain alloys, butquestions remain as to whether they introduce hydrogen embrittlementproblems for Ti foil or honeycomb core substrates. Our preferredcleaning and activation processes are prewetting, steam cleaning,alkaline etching to activate the surface, and BOECLENE or other acidetching (i.e. H₂SO₄—HNO₃-ammonium bifluoride) to desmut titanium alloys.

The drying cycle for the sol coating is another significant processingparameter to controlling adhesive bond performance. The drying cycleincludes: (1) ambient air flash time after application of the sol; (2)oven dry time at temperature; and (3) storage time in air thereafterprior to application of the primer. As shown in FIG. 8, shorter dryingtimes at 230° F. tended to yield better results. An oven drying time of15-30 minutes at 140-230° F. in air lead to better hot/wet durability.

FIG. 9 shows a difference we observed in performance arising fromapplying the sol by spraying or dipping. In this experiment, the sol wassprayed onto the substrates using a high velocity, low pressure (HVLP)spray gun. A coat consisted of light, but complete, coverage of thesurface. The coating was allowed to flash dry between coats. In allcases, the sprayed coatings did not perform as well as the dippedcoatings. Specimens sprayed with an even number of coats did not performas well as specimens sprayed with an odd number of coats, this effectmay be an artifact of how the gradient coating layered onto the surface.In applying an even number of coats, the GTMS may couple with the nextlayer's glycidoxy end of the GTMS in the next layer, the silica orientedaway from the metal surface where it-cannot bond with the metal andwhere it interferes with the sol-gel film/primer interfacial chemistry.Consequently, there would be fewer organic functionalities-available forbonding to the resin. With an odd number of coats, a glycidoxy edgewould occur at the outer surface if this intermediate reaction occurs.Hence, we suspect, we achieve better performance. This gradient effecthas been seen in multilayers of phospholipids and in other biochemicalsystems.

We also examined a drench method for applying the sol, which combineselements of both the dip and spray processes. In this applicationtechnique, the surface of the part is wetted with a continuous stream ofsolution for a given period of time. The solution surface is wet withthe solution for longer than in the spray process, but not as long asthe dip process. One of the advantages of this technique is that it doesnot require the precise skills of an expert sprayer. It also usessignificantly less solution than the dip (immersion) process. Thecoating thicknesses are controlled by the coating formulation itself andlength of time that the surface is wet.

Our testing on epoxy systems was primarily conducted usingsolvent-based, chromated primer Cytec BR127. We conducted tests with thenonchromated water-based Cytec XBR 6757 primer. The water-based primerwith a preferred sol-gel formulation yielded comparable results to thesolvent-based primer.

Titanium samples (Table 2) were aqueous degreased, then grit blastedusing 180 grit alumina abrasive powder, and treated with sol using a dipcoating. TABLE 2 Titanium Specimens Prepared For Epoxy AdhesionPerformance Testing Sample Sample Size # of Surface ID (inches) PanelsTest Treatment Primer GAB01 6 × 6 × 0.125 4 Wedge Crack GTMS/TPOZ BR 127Extension GAB02 6 × 6 × 0.125 4 Wedge Crack GTMS/TPOZ Cytec 6757Extension GAB03 6 × 6 × 0.050 2 Wedge Crack GTMS/TPOZ BR 127 ExtensionGAB04 6 × 6 × 0.050 2 Wedge Crack GTMS/TPOZ Cytec 6757 Extension GAB05 4× 7 × 0.063 12 Lap Shear, GTMS/TPOZ BR 127 Various Conditions GAB06 4 ×7 × 0.063 8 Lap Shear, GTMS/TPOZ Cytec 6757 Various Conditions GAB07finger panels 6 Lap Shear, GTMS/TPOZ BR 127 Various Conditions GAB08finger panels 6 Lap Shear, GTMS/TPOZ Cytec 6757 Various ConditionsGAB010 6 × 6 × 0.125 4 Wedge Crack CAA/5V BR 127 Extension GAB011 6 × 6× 0.125 12 Lap Shear, CAA/5V BR 127 Various Conditions

The chromic acid anodize (CAA) specimens displayed 100% cohesivefailure. The failure modes of the sol-gel panels varied. Panels primedwith the solvent-based BR127 tended to show between 10-50% cohesivefailure and the remainder adhesive failure at the adherent-solinterface, while the panels primed with the water-based XBR 6757exhibited from 80-100% cohesive failure.

The results of crack growth tests during long term exposure to 140° F.condensing humidity are shown in FIG. 10. H133-2 is a sol that had beenaged for 1.9 hrs, while H133-3 had aged for 0.2 hrs prior toapplication. These solutions produced coatings having crack growthcomparable with the chromic acid anodize standards even after 2000 hourshot/wet exposure. The crack growth rate had leveled off at approximately0.1 inches crack extension. Abrupt jumps in the data are due to thedifficulty in visually measuring minute changes in the crack extension.Ideally, a smooth curve could be drawn into the raw data representingthe growth rate over time. These results show that the hot/wetdurability of these water-based sol coatings compares well with theCAA-controls.

Lap shear data were collected on an Instron Series IX AutomatedMaterials Testing System 6.04. The sample rate was 9.1 pts/sec with acrosshead speed of 0.05 in/min. Lap shear data is listed in Table 3.Results are an average of five finger test specimens per data point.TABLE 3 Lap Shear Data CAA Control Boeing SG Boeing SG with Cytec BR 127with Cytec BR 127 with Cytec XBR 6757 Ultimate Stress Failure ModeUltimate Stress Failure Mode Ultimate Stress Failure Mode (psi) (% coh)(psi) (% coh) (psi) (% coh) −65° F. 7988 80 7900 100 8822 100 Lap ShearRT 5935 100 6278 100 6015 100 Lap Shear 180° F. 3722 100 4123 100 3586100 Lap Shear

The lap shear failure mode is predominantly cohesive within the adhesivelayer in all of the specimens at all of the temperatures. The data forthe CAA control and the sol-gel surface preparations with both primerswas essentially the same within experimental error.

Lap shear coupons produced with a sol that had been aged for 150 hrs orone that had been aged for 338 hrs had essentially the same ultimatestress values. Exposure of the lap shear specimens made with sol in ahot/wet environment resulted in a 14% decrease in ultimate stress after168 hrs and 27% after 500 hrs. A 500 hour hot/wet exposure of the CAAcontrol specimens resulted in a 7% degradation.

Electronic Spectroscopy for Chemical Analysis (ESCA) studies conductedon an ethanol-based formulation revealed the mode of failure-duringhot/wet exposure. The bonding surfaces of two previously tested wedgecrack sample coupons were examined using ESCA analysis techniques. Thefirst sample, H83KAK-1, was a grit blasted Ti-6-4 panel which had beencoated with an isopropanol solution of 2% GTMS, 1% TPOZ and 1%80%-acetic acid. It was tested at 120° F. and 95% relative humidity for864 hours and had an average increase in the initial crack length of0.45 inches by the end of the test.

The sample was split in half and the smaller mating surface section fromadhesively failed areas of the coupon were examined. The bare titaniumsurface of the adhesively failed sample section had high carbon, lowtitanium, and small amounts of silicon and zirconium. As this surfacewas sputter-etched, the percentages of silicon, zirconium, and chromiumincreased and reached a maximum before dropping off with continuedsputtering. After 300 Å of sputter etching, the carbon content hadreached a minimum and the titanium, aluminum, and vanadium content hadreached a maximum. At this depth, the zirconium content was about 20%below its maximum value and the silicon content was more that 50% belowits maximum value. Table 4 shows the sputtering data for this system.TABLE 4 ESCA Sputter Data for Failed Surfaces adhesive metal atom(sputter) metal metal sputter O 40.97 28.23 36.88 31.52 Ti 2.62 22.74 C36.89 59.61 44.36 19.06 Al 4.05 2.99 6.12 14.16 Si 11.9 4.45 3.42 5.38Zr 2.81 0.51 0.63 2.34 Ar 2.2 N 1.1 0.91 0.95 1.45 V 1.15 Cr 2.29 1.431.82Aging studies with ESCA suggest that aging the solution does not alterthe surface characteristics significantly.

Roughness of the etched titanium surface prevented accuratedetermination of the thickness of the sol coating. ESCA measures an areaof the surface approximately 600 μm in diameter. Table 4 shows thesurface composition following various sputter times on the coatedsample. The etched titanium surfaces have “craters” approximately 15 μmin diameter and 2-5 μm deep. Therefore, the ESCA experiment will measureabout 20 “craters” and associated ridges. The sol coating is likely tobe thinner on the ridges than in the craters, perhaps thin enough forthe substrate material to have a measurable signal even withoutsputtering.

The ESCA data for both samples in Table 5 show a small amount oftitanium at the surface indicating that some areas of the substrate arenot coated or that sol coating on the ridges is thinner than about 130Å. Argon plasma sputtering the surface gradually removes the solcoating, as indicated by the decrease in Si and Zr, but there is not asharp change in surface composition. The data are consistent with asurface having roughness greater than the coating thickness. TABLE 5ESCA Data for Unexposed Water-based Sol-Gel Specimens 24 hr old solution1 hr old solution 345 Å 430 Å 430 Å Light Sput- Sput- Sput- Atomic %Surface Sputter tered tered Surface tered Carbon 44.8 19.0 5.2 5.5 57.523.0 Oxygen 40.6 52.8 23.0 23.5 32.1 31.4 Titanium 3.1 9.2 51.0 54.0 2.431.3 Aluminum 0.6 2.4 9.8 8.3 0.6 5.8 Vanadium — — 2.8 2.4 — 1.7Zirconium 2.0 3.0 0.7 1.0 0.8 0.5 Niobium 0.5 1.0 0.3 — 0.3 — Molybdemun0.2 0.5 0.2 — 0.2 — Fluorine — 0.8 — — — — Silicon 8.1 10.8 — — 3.5 —Nitrogen — — 3.3 1.8 1.1 2.4 Copper — — — — 0.3 0.3 Calcium — — — — 1.21.2 Argon — 0.4 3.7 3.4 — 2.4

The theoretical composition of the fully hydrolyzed sol-gel film is 4parts SiO_(1.5)Gly and 1 parts ZrO₂ where Gly is the glycidoxy groupattached to the silicon. The (silicon+zirconium):carbon:oxygen ratio fora homogeneous coating formed from this sol is 1:4:4:3.2. Theexperimental value for the surface of the 24 hr old formulation specimenshows the (silicon+zirconium):carbon:oxygen ratio to be approximately1:4:5:4.0, close to the theoretical value. The ratio of (Si+Zr):C:Oratio becomes 1:1.2:3.8. Continued sputtering further reduces the carbonsignal to near background levels and the oxygen signal decreases aswell. We interpret the data to indicate that the deposited film is nothomogeneous. The glycidoxysilane is located primarily on the surfacewith the coating composition changing to zirconia and titania andfinally the metallic substrate. This measured gradient is consistentwith our model of formation of films from our sols and observations ofincreasing water aversion of substrates as coatings are deposited. Thethickness of the sol coating is not accurately determined by thismeasurement but it appears to be a minimum of 100-300 Å. Considering theescape depth of ESCA being about 100 Å, the glycidoxy-rich surface layeris no more than about 75-150 Å thick as indicated by the decrease in thecarbon level.

The low carbon level in the coating after 450 Å etch indicates thathydrolysis of the sol is essentially complete within 24 hours. Data forcoatings deposited from the same sol aged for only 1 hour showsignificantly greater amounts of carbon both at the surface and at the450 Å level. Incomplete hydrolysis will leave alkoxy groups attached toboth silicon and zirconium. In addition, acetate groups from theincomplete hydrolysis of the stabilized zirconate will also beincorporated throughout the coating. The ESCA data do not differentiatebetween acetate, glycidoxy, and alcohol carbon and oxygen.

We also conducted experiments using aluminum substrates (alloys 7075 and2024). FIG. 3 shows the cumulative crack growth or extension as afunction of time for an epoxy adhesive. Crack growth was the smallestfor a sol coated 7075 alloy. The hot/wet durability of the sol coatedspecimens was comparable with the phosphoric acid anodized (PAA)controls for 1000 hrs of testing. The sol coated specimens wereacceptable as measured by BAC 5555 PAA requirements, so the sol coatingis an alternative and improvement to PAA for at least these aluminumalloys.

We collected lap shear data as well for the sol coated 7075 and 2024alloys. Room temperature (RT) results surpass the minimum thresholdspecified in BAC 5555. Our tests indicate that specimens primed with awaterbased nonchromated primer (Cytec XBR 6757) performed as well as orbetter than specimens primed with the conventional chromated,solvent-based primer (Cytec BR 127). Again, 7075 alloys had betterperformance than 2024 alloys. Floating roller peel test data (reportedas the average of five individual specimens) showed that sol coatedaluminum specimens compared favorably with the bond strengths achievedwith conventional PAA controls.

Drenching v. dipping for aluminum substances has little effect on thebond strength. This fact makes the sol coating desirable for fieldrepair and depot maintenance because specialized equipment likely is notrequired to obtain the benefits of sol coating. Also, the data wecollected suggests that the coating process is robust rather than a“craft.” We also found little effect from drenching versus mistspraying.

Parameters such as concentration, acid catalyst, aging,hydrolysis/concentration, and the ratio of the reactants will need to beoptimized for large scale and spray operation. We anticipate that thesewill be different than those optimized for dipping. The use ofsurfactants and thixotropic agents in the solution may improve the spraycharacteristics of the solution, but may adversely affect the bondingperformance. These agents may help to provide a more uniform sprayedcoating and improve the manufacturability of the process.

There are fundamental differences in the manner of film formationbetween spraying and dipping. With dipping or immersion coating, thethermodynamically favored products of slower reactions can dominate.With the part immersed in the solution, reactant can reach to the metalsurface through mechanical, thermal, and mass transport mechanisms.Reaction products can diffuse away from the surface. The mostthermodynamically stable coating will develop. In the case of spraying,only a thin film of the sol contacts the surface. Depletion of reactantscan and likely does occur as the sol flows down the surface.Consequently, reaction products build up and may influence the chemistrythat occurs. In addition, reaction products remain on the surface whenthe film dries. The sol-gel film developed with spraying is dominated bykinetically accessible products. Advantages of spraying include coatingthickness control and uniformity.

The preferred zirconium compounds for making the sol are of the generalformula (R—O)₄ Zr wherein R is lower aliphatic having 2-5 carbon atoms,especially normal aliphatic (alkyl) groups, and, preferably,tetra-n-propoxyzirconium, because of its being readily availablecommercially. We believe that branched aliphatic, alicyclic, or arylgroups would also perform satisfactorily. For applications involvingextended exposure to hot/wet conditions, we want the organo moiety onthe zirconium to have thermo-oxidative stability. Correspondingorganometallics, especially a titanium alkoxide or an aluminum alkoxide,might be used together with or in place of the zirconium compound.

The preferred organosilane compounds (available from Petrarch or Read)for making the sol are:

-   -   3-aminopropyltriethoxysilane,    -   3-glycidoxypropyltrimethoxysilane,    -   p-aminophenyltrimethoxysilane,    -   m-aminophenyltrimethoxysilane,    -   allyltrimethoxysilane    -   n-(2-aminoethyl)-3-aminopropyltrimethoxysilane    -   3-aminopropyltrimethoxysilane    -   3-glycidoxypropyldiisopropylethoxysilane    -   (3-glycidoxypropyl)methyldiethoxysilane    -   3-glycidoxypropyltrimethoxysilane    -   3-mercaptopropyltrimethoxysilane    -   3-mercaptopropyltriethoxysilane    -   3-methacryloxypropylmethyldiethoxysilane    -   3-methacryloxypropylmethyldimethoxysilane    -   3-methacryloxypropyltrimethoxysilane    -   n-phenylaminopropyltrimethoxysilane    -   vinylmethyldiethoxysilane    -   vinyltriethoxysilane or    -   vinyltrimethoxysilane.

In these organometallics, the organo moiety preferably is aliphatic oralicyclic, and generally is a lower n-alkoxy moiety having 2-5 carbonatoms. The organosilane includes typically an epoxy group (for bondingto epoxy or urethane resins or adhesives) or a primary amine (forbonding to polyimide resins or adhesives).

If the sols are alcohol-based, the preferred alcohols are ethanol,isopropanol, or another lower aliphatic alcohol.

The sols can be used to make sol-gel films on the following aluminum andtitanium alloys: Al 2024; Al 7075; Ti-6-4; Ti-15-3-3-3; Ti-6-2-2-2-2;and Ti-3-2.5. The sol coating method can also be used with copper orferrous substrates, including stainless steel or an Inconel alloy.

3. Hybrid Laminates

Sol coated metals are useful in hybrid laminates like those described inU.S. Pat. No. 4,489,123. These hybrid laminates are candidates for useas aircraft skin panels and other structural applications in subsonicor, especially, supersonic aircraft. The utility of these hybridlaminates hinges on a sound, strong adhesive bond between the metal andresin. The sol coating of the present invention provides a high strengthadhesion interface at relatively low cost compared with conventionalalternatives in a reasonably simple manufacturing process.

Hybrid laminates should have a high modulus (absolute strength) and befatigue resistant so that they have long life. They should exhibitthermomechanical and thermo-oxidative stability, especially in hot/wetconditions. They should have a high strength-to-weight ratio whilehaving a relatively low density as compared to a solid (monolithic)metal. They should be damage resistant and damage tolerant, but theyshould dent like metal to visibly show damaged areas long before thedamage results in actual failure of the part. They should be resistantto jet fuel and aerospace solvents. Finally, they should be resistant tocrack growth, preferably slower than monolithic titanium.

The hybrid laminates generally have alternating layers of titanium alloyfoil 110 and a fiber-reinforced organic matrix composite 120 (FIG. 11).The foil typically is sol coated in accordance with the method of thepresent invention to enhance adhesion between the foil and the matrixresin of the composite (and any intervening primers or adhesives). Thesol coating may also provide corrosion resistance to the titanium. Thefoil typically is about 0.01-0.003 inches thick (3-10 mils) ofβ-annealed titanium alloy having a yield strain of greater than about1%. The composite typically is a polyimide reinforced with high strengthcarbon fibers. The polyimide is an advanced thermoplastic orthermosetting resin capable of extended exposure to elevatedtemperatures in excess of 350° F., such as BMI, PETI-5, PIXA, KIIIB, ora Lubowitz and Sheppard polyimide. The composite is one or more plies toprovide a thickness between the adjacent foils of about 0.005-0.03inches (5-30 mils). Other configurations of the hybrid laminates, likemetal reinforced resin composites, might be used.

The preferred composite is formed from a prepreg in the form of a tow,tape, or woven fabric of continuous, reinforcing fibers coated with aresin to form a continuous strip. Typically, we use a unidirectionaltape. The fibers make up from about 50 to 70 volume percent of the resinand fibers when the fiber is carbon, and from about 40 to about 60volume percent when the fiber is boron. When a mixture of carbon andboron fibers is used, total fiber volume is in the range 75 to 80 volumepercent. The plies may be oriented to adjust the properties of theresulting composite, such as 0°/90° or 0°/−45°/+45°/0°, or the like.

Hybrid laminates of this type exhibit high open-hole tensile strengthand high compressive strength, thereby facilitating mechanical joiningof adjacent parts in the aircraft structure through fasteners. Thelaminates might also include Z-pin reinforcement in the composite layersor through the entire thickness of the laminate. Z-pinning techniquesare described in U.S. Pat. Nos. 5,736,222; 6,027,798; 5,980,665;5,862,975; or 5,869,165.

The hybrid laminates can be used in skin panels on fuselage sections,wing sections, strakes, vertical and horizontal stabilizers, and thelike. The laminates are generally bonded as the skins 100 of sandwichpanels that preferably are symmetrical and include a central core 130 oftitanium alloy honeycomb, phenolic honeycomb, paper honeycomb, or thelike (FIG. 12), depending on the desired application of sandwich panel.Sandwich panels are a low density (light weight), high strength, highmodulus, tailorable structure that has exceptional fatigue resistanceand excellent thermal-mechanical endurance properties.

The hybrid laminates are also resistant to zone 1 lightning strikesbecause of the outer titanium foil.

Outer metal layers protect the underlying composite from the most severehot/wet conditions and the cleaning solvents that will be experiencedduring the service life of the product, especially if it is used onsupersonic aircraft.

To prepare the hybrid laminates, generally we pretreat cleaned,β-annealed Ti-6Al-4V alloy foils in various concentrations (i.e. 20%,60%, or 80%) of TURCO 5578 alkaline etchant (supplied by Atochem, Inc.of Westminster, Calif.). After water rinsing the foils are immersed in35 vol % HNO₃—HF etchant at 140° F. to desmut the foil, they are rinsedagain, and, then, are sol coated.

For the strongest interaction between the sol-gel film and thecomposite, we prefer that the organic moiety of the silane correspondwith the characteristics of the resin. For example, PETI-5 is a PMR-typeor preimidized, relatively low molecular weight resin prepreg havingterminal or pendant phenylethynyl groups to promote crosslinking andchain extension during resin cure. Therefore, the silane coupling groupmight include a reactive functionality, such as an active primary amine;an anhydride, carboxylic acid, or an equivalent; or even a phenylethynylgroup, to promote covalent bonding between the sol coating and theresin. The organic moiety might simply be an aliphatic lower alkylmoiety to provide a resin-philic surface to which the resin will wet orhave affinity for to provide adhesion without producing covalent bondsbetween the organosilane coupling agent and the resin. The aliphaticmoiety, however, would still provide hydrogen atoms for hydrogen bondingwith the numerous heteroatoms (oxygen) in the cured PETI-5 imide. If theresin includes a nadic or maleic crosslinking functionality, as Lubowitzand Sheppard suggest or as occurs in bismaleimides, then theorganosilane coupling agent might include the same nadic or maleiccrosslinking functionality or an amine, —OH, or —SH terminal group forcovalent bonding through capping extension or the Michael's additionacross the active unsaturated carbon-carbon bond in the resin's cap. Forhigher temperature applications, we recommend using an aromaticorganometallics since these compounds should have higherthermo-oxidative stability.

Greater covalent interaction between the resin and sol-gel film islikely to occur if the resin is a PMR formulation at the time of layuponto the foil rather than a fully imidized resin of relatively highformula weight, such as Lubowitz and Sheppard propose. Of course, PMRformulations have their processing limitations. Knowing which resinapproach will provide the best overall performance in the hybridlaminates remains for further testing, as does selection of the absolutetype of resin and its formulation. Ideally, the resin is easy to processbecause it has few adverse aging consequences from extended exposure toambient conditions common during fiber placement. The resin prepregsshould also have long shelf lives. Alternately, the resin should besuitable for in situ consolidation while being placed on the foil.

Tows of mixed carbon and boron fibers suitable for these hybridlaminates are sold under the tradename HYBOR by Textron SpecialtyMaterials of Lowell, Mass. Boron fibers provide high compressivestrength, while carbon fibers provide high tensile strength. Thepreferred boron fiber has the smallest diameter (typically 4-7 mils) andthe highest tensile elongation.

Hybrid laminates can have open-hole tensile strengths of about 150-350ksi and an ultimate tensile strength in excess of 2×10⁶ psi/lb/in³.

4. Paint Adhesion

A sol coating is particularly useful as an adherent for surface coatings(paints), especially urethane coatings, that are common in aerospaceapplications. Essentially the same aspects that make the sol coatingsadvantageous for hybrid laminates make them advantageous for paintadhesion. They convert the metal surface into a surface with highaffinity for the paint binder. They preferably include components thatcovalently bond to the metal substrate as well as to the paint binder.In this application, rather than applying an adhesive 33 (FIG. 13) overthe primer 39, we apply the exterior surface coating or paint 55 (FIG.15) typically pigments carried in a urethane binder. The sol coatingprovides long lasting durability for adherence of the primer and finishcoating to the metal.

The sol coating of the present invention promises to improve paintadhesion and to simplify field repair and maintenance of painted metalsurfaces, especially titanium or aluminum structure painted with epoxyor urethane paints. Because the sol has essentially a neutral pH, it canbe easily used in the field for touch up without the precautions orspecial equipment necessary for andozing. Through the sol coating 45,the paint 55 is essentially covalently bonded to the metal 65 as shownin FIG. 15 and adhesion is significantly enhanced. We obtain improvedadhesion even for parts that have had acid surface etching several weeksprior to priming and painting.

The preferred sol coating process for paint adhesion improvementinvolves:

-   -   (a) cleaning the surface with an aqueous detergent or another        cleaning solvent.    -   (b) wetting the surface for at least five (5) min prior to        applying the sol;    -   (c) applying the sol by spraying or another suitable means while        continuing to keep the surface wet for 1-2 min;    -   (d) drying the sol to form a sol coating at ambient conditions        for 15-60 min;    -   (e) heating the sol coating to a temperature in the range from        160-250° F. (preferably, 230±20° F.) for 15-45 min to complete        the drying; and    -   (f) applying primer or paint, as appropriate, between 2-72 hours        after completing drying steps (a) & (e).

The sol has a pot life of up to about ten hours. Of course, there is aninduction time following mixing that reduces the period of time in whichthe sol can be applied. We achieve the most consistent results usingdeionized water as the carrier or solvent. The deionized water shouldhave a minimum resistivity of about 0.2 M. The sol might also be used asa coupling agent in polyimide resin composite bonds (especially BMI orKIIIB composites) to other resin composite parts or to metals.

The sol coating can be applied to advantage on sheet, plate, foil, orhoneycomb. While described primarily with reference to Ti, the solcoating is also useful on Al; Cu, or Fe pure metals or alloys. Oneapplication for copper includes coating a susceptor mesh or foil used inthermoplastically welded composite structures. Another application forcoated copper includes protecting the metallic interlayers in multichipmodules or multilayer chip module packaging. Suitable ferrous alloys aremild steel, cold rolled steel, stainless steel, or high temperaturealloys, such as the nickel-iron alloys in the INCONEL family.

To prepare the sol according to the mixing procedure outlined in Table6. The absolute volume mixed can vary as needed. Once mixed the sol mayage for 4-6 hours to reach equilibrium. TABLE 6 Sol Preparation forAmino-based Silanes STEP Stir 500 ml deionized (DI) water in 1000 mlFlask 1 (1000 ml) flask Add 4 Drops NH₄OH Verify pH around 7-8 (add moreif appropriate) Add 7.3 ml glacial acetic acid (GAA) to 50 ml Flask 2(50 ml) flask Add 10 ml TPOZ into 50 ml flask with GAA Shake mixture Add34 ml organosilane to 1000 ml flask, avoid Flask 1 (1000 ml) drops onsides of flask Cover flask Stir for 20 to 30 minutes Add about 300 ml DIto 500 ml flask Flask 3 (500 ml) Add about 200 ml DI to 200 ml flaskFlask 4 (200 ml) Dilute contents of 50 ml flask with equivalent Flask 2(50 ml) volume of DI water Add entire contents of 50 ml flask to 500 mlFlask 2 (50 ml) + flask (should be clear) Flask 3 (500 ml) Add 3 ml ofNH₄OH, squirt in while, agitating violently; pH should be about 5 (milkywhite) Add contents of 500 ml flask to 1000 ml flask Flask 3 (500 ml) +Flask 1 (1000 ml) rinse 500 and 50 ml flasks with DI water from Flask 1(1000 ml), 200 ml flask (pH between 8-9) into 1000 ml Flask 2 (50 ml)flask Flask 3 (500 ml), Flask 4 (200 ml) Allow solution to age for up to4-6 hours under constant agitation prior to application

A presently preferred surface pretreatment for the metal (albeit onedifferent from that schematically illustrated in FIG. 1) includes thesteps outlined in Table 7. TABLE 7 Surface Treatments PretreatmentProcess Steps Temp Time Aqueous Degrease with Super 150 ± 5 F. 20 to 30minutes Bee per BAC 5763 (optional) Water immersion rinse  100 ± 15 F. 3to 5 minutes (optional) Alkaline Clean Brulin 815 GD 140 ± 5 F. 20 to 40minutes per BAC 5749 Water immersion rinse  100 ± 15 F. 3 to 5 minutesWater Spray Rinse Ambient NA Turco 5578 Alkaline Etch 190 ± 5 F. 15 to20 minutes (80% concentration) DI Water Immersion Rinse Ambient 3 to 5minutes HNO₃ Desmut (35% 150 ± 5 F. 3 to 4 minutes concentration) DIWater Immersion Rinse with Ambient 3 to 5 minutes Agitation DI WaterSpray Rinse Ambient NA Verify parts are water break free for greaterthan 60 secondsBOECLENE desmutting can replace HNO₃. The composition and use ofBOECLENE is described in U.S. Pat. No. 4,614,607, which we incorporateby reference. Acid etching might use other etchants than HNO₃—HF₁, butwe prefer that etchant.5. Pigmented Sols

While our preferred sol has GTMS and TPOZ in concentrations of about3-30 vol % (0.03-0.3M), we might also includeaminopropyltrimethoxysilane to tailor the coating for adhesion topolyimide overcoats or to help disperse the organic, metallic, metalsulfide, or metal oxide pigments that we add to obtain a desired gloss,color,-reflectivity, conductivity, emissivity, or combination thereof.We might also add a titanium alkoxide-or an aluminum alkoxide, liketitanium isopropoxide or aluminum butoxide, in combination with the TPOZor in place of the TPOZ to provide desired characteristics for thecoating.

Generally the molar (or volumetric) ratio of GTMS:TPOZ is about 3.7:1,but the optimum ratio depends on the application for the coating.

The coating thickness increases as the concentration of theorganometallics (or reagents) of the sol increases. In addition, theease of application and the rate of build up of the coating increases asthe organometallics concentration increases. At dilute concentrations,many coats might be required. Runs and drips are difficult to controlbecause of the low concentration of organometallics. Concentrated sols,however, may exhibit poor adhesion to the substrate, especially when thecoating is sprayed on. For spraying, we prefer to limit theorganometallics concentration to about 15-30 vol % (0.15-0.30M). Ourgoal is to provide a sol-gel replacement for paint that will have highimpact resistance and be usable over a wide range of temperatures.

To minimize the proportion of organics in the coating, we can substitutetetra-ethyl-orthosilicate (TEOS) for all or part of the GTMS or otherorganosilane, first hydrolyzing it in alcohol. We can make gradedsol-gels by adjusting the ceramic character as we previously described.

The concentration of pigments we can add to the sol is also directlyproportional to the organometallics concentration. Also, because thecoating is very thin (on the order of nanometers), the pigments shouldhave comparable or smaller characteristic dimensions. The molar ratio ofpigments to organometallics should be in the range from about 1-3 partspigment to 1 part organometallics. The concentration of pigments alsodepends on the size and physical and chemical characteristics of thepigments. Their total wetted surface area appears to be an importantconsideration. The sols might also include carbon or graphite particlesor fibers.

Generally the pigments are metal flakes, metal oxide particles, ororganometallic particles. Suitable aluminum flake pigments include theAquasil BP series of pigments available from Siberline Manufacturing Co.The pigments might be glass, mica, metals (like nickel, cobalt, copper,bronze, and the like available from Novamet) or glass flake, silvercoated glass flake, mica flake, or the like available from PottersIndustries, Inc. These flakes typically are about 17-55 μm for theircharacteristic dimension. In some applications, ceramic pigments may beappropriate. Of course, the pigments can be mixed to provide the desiredcharacteristics for the coating.

We have had success spraying with a Binks Mach 1 HVLP spray gunpigmented sols that included Titanox 2020 titanium oxide pigment(available from NL Industries), copper oxide or iron oxide pigments(available from Fischer Scientific), or NANOTEK titania, zinc oxide, orcopper oxide pigments (available from Nanophase TechnologiesCorporation). These pigments are generally spherical with diameters inthe range form about 30 nm (for the NANOTEK pigments) to micron sizes.

Our pigmented sol coatings pass wet and dry tape adhesion tests and theGE 80 in-lb impact test on 2024 aluminum, as required for enamelcoatings per Boeing Material Specification (BMS) 10-60K “ProtectiveEnamel.” The coatings pass the impact test even at liquid nitrogentemperatures (about −196° C. (−321° F.)) and after thermal cycling (orextended exposure) to about 750° F. in air. In fact, in one impact test,the substrate failed rather than the coating after we heated the samplefor one hour at 1000° F. Coatings using powder pigments are better inimpact tests than those that use flakes. The differences in impact testresults may arise from the high aspect ratios (length/width)characteristic of flakes, but may also reflect our failure to optimizethe sol formulations and spray gun settings for the sols we have testedto date.

The pigmented coatings should also satisfy Boeing Material SpecificationBMS 14-4H “Protective Coating, Inorganic, Heat, Weather and OilResistant,” if the coated products are targeted for use in aerospaceapplications. Unlike the traditional coatings qualifying to BMS 14-4H,the pigmented sol coatings of the present invention are significantlythinner and, thereby, do not impose as significant a weight penalty onthe final product.

Other suitable pigments include those described in U.S. patentapplication Ser. No. 08/770,606 entitled “High Efficiency MetalPigments” or U.S. Provisional Patent Application 60/089,328 filed Jun.15, 1998, entitled “Method for Making Particulates of ControlledDimensions.” Other potential pigments are described in the CRC HANDBOOKOF CHEMISTRY AND PHYSICS, 51^(st) ed., F-60 through F-62 (1970).

Our method to prepare the sol alters the reaction kinetics fromtraditional sols and results in sol-gel coatings. We achieve arelatively long effective pot life for the sol as opposed to processeslike Yoldas uses in U.S. Pat. No. 4,754,012. In fact, the pot life ofour sol is comparable to conventional aerospace catalyzed paints (4-8hours) rather than 0.5-1.0 hour. Typically our pot lives are from 0.5-6hours, but we have successfully applied sols as old as 24 hours. Webelieve that the sol-gel's adhesion degrades if the pot life isexcessive. We have yet to measure the adhesion for the sol-gel formedwith a 24 hour old sol.

Alcohol-based sols present fire hazards. They are regulated today andare likely to be prohibited at some in the future. Our water-based solscan be applied wherever it is convenient, because they are lowvolatile-omitting formulations. Alcohol-based sols require specialhealth and safety equipment. The waterborne sols do not require aspecialized paint spray booth.

Conductive coatings have promise for space vehicles to preventelectrostatic discharges grounding, and electromagnetic interference(EMI) problems. In a space environment, the preferred sol-gel coatingprovides resistance to atomic oxygen, ultraviolet radiation, and/or highenergy particles that comnmonly erode unprotected substrates, especiallycomposites, in low earth orbit (LEO). Such a coating might be thepigment Zr—Si sol or a graded sol using TEOS in one or more layers.

While we have described preferred embodiments, those skilled in the artwill readily recognize alterations, variations, and modifications whichmight be made without departing from the inventive concept. Therefore,interpret the claims liberally with the support of the full range ofequivalents known to those of ordinary skill based upon thisdescription. The examples illustrate the invention and are not intendedto limit it. Accordingly, define the invention with the claims and limitthe claims only as necessary in view of the pertinent prior art.

1. A sol suitable for surface treatment of a substrate to provide a hightemperature coating, made by mixing in a suitable carrier: a) aneffective amount of an organometallic alkoxide containing either Ti orAl, for covalently bonding to the substrate; b) an effective amount ofan organosilane coupling agent for forming a sol-gel network with thealkoxide; c) an organic acid to catalyze the networking of theorganosilane to the alkoxide and to stabilize the rate of hydrolysis ofthe alkoxide; and d) an effective amount of a pigment to control gloss,color, reflectivity, electrical conductivity, emissivity, or acombination thereof of the substrate when coated with the sol to form asol-gel thin film finish.
 2. The sol of claim 1 wherein the pigmentincludes metal flakes.
 3. The sol of claim 2 wherein the flakes includealuminum.
 4. The sol of claim 1 wherein the carrier is water.
 5. Agelled sol of claim
 1. 6. The sol of claim 1 wherein the volumetricratio of alkoxide:organosilane:pigment is about one part alkoxide:atleast two parts organosilane:about one-three parts pigment by volume. 7.A graded mixed metal Zr:Si sol-gel, comprising: (a) a first layer madeby mixing in a suitable carrier an effective amount of anorganozirconium with TEOS to form a sol-gel network; and (b) a secondlayer made by mixing in a suitable carrier an effective amount of theorganozirconium and an organosilane selected from the group consistingof: 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane,p-aminophenyltrimethoxysilane, m-aminophenyltrimethoxysilane,allyltritnethoxysilane, n-(2-aminoethyl)-3-aminopropyltrimeethoxysilane,3-arninopropyltrimethoxysilane,3-glycidoxypropyldiisopropylethoxysilane,(3-glycidoxypropyl)methyldiethoxysilane3-glycidoxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane,3-mercaptopropyltriethoxysilane,3-methacryloxypropylmethyldiethoxysilane,3-methacryloxypropylmethyldimethoxysilane,3-methacryloxypropyltrimethoxysilane,n-phenylaminopropyltrimethoxysilane, vinylmethyldiethoxysilane,vinyltriethoxysilane, vinyltrimethoxysilane, and mixtures thereof toform a sol-gel network, wherein the first layer has a stronger ceramiccharacter than the second layer, and further comprising anorganometallic alkoxide containing Ti or Al in at least one of the firstlayer and the second layer.
 8. A method for providing protection to asubstrate, comprising the step of: coating the substrate with thesol-gel of claim
 1. 9. The sol of claim 1 wherein the alkoxide includesan effective amount of titanium isopropoxide.
 10. The sol of claim 1wherein the alkoxide includes an effective amount of aluminum butoxide.